1. Technical Field
The present disclosure relates to an audio system and, more particularly, to an audio system having an improved signal to noise ratio (SNR) and a method of controlling an audio system.
2. Discussion of Related Art
Audio amplifiers are typically classified as either class-A, class-B, class-AB, or class-D according to a driving circuit of the audio amplifier known as the output stage.
In the class-A output stage, bias voltages are applied to output transistors in a mute state, in which no signal is output, and a bias current flows through the output transistors in the mute state. Thus, the class-A output stage has high thermal dissipation and low thermal efficiency. The class-B output stage is configured to prevent the bias current in the mute state. Significant crossover distortion, however, is caused in the class-B output stage when the output signal passes through a reference voltage, that is, when the output transistors are turned off. The class-AB output stage is configured to have a small bias current in the mute state to achieve the lower distortion of the class-A configuration and the higher efficiency of the class-B configuration. The crossover distortion, however, still exists and a high-capacity heat sink is required in the class-AB output stage.
The class-D output stage includes transistors operating as an ON/OFF switch. A turn-on resistance of the transistors is relatively small, and thus the class-D output stage has a high efficiency. According to an IEC (International Electrotechnical Commission) standard, the class-D amplifier is defined as any amplifier, “in which the current in each active device supplying the load is switched from zero to a maximum value by a carrier signal, modulation of which conveys the useful signal.” Accordingly the class-D output stage includes all of the output stages, that is, the driving circuits are operated by a switching amplification regardless of whether an analog input or a digital input is received and regardless of the types of the internal signal processes used. The audio amplifiers including such a class-D output stage or a class-D driving circuit are referred to as digital amplifiers or digital amps.
Because of superior characteristics of the class-D amplifier, numerous audio systems reproduce sounds from audio signals by using the class-D amplifier. Recently, according to demands of users, audio systems generally include at least two amplifiers for playing stereo audio signals.
FIG. 1 is a diagram illustrating a system 100 for recording and playing a stereo audio signal.
In a device 100a for recording stereo audio signals, at least two microphones 115a and 115b receive sounds from sound sources 110a and 110b. A first amplifier 120 amplifies the received sounds and a recorder 130 records the amplified sounds and stores the recorded sounds in a storage device 100b, such as a compact disc (CD).
In a device 100c for playing the recorded stereo audio signals, a player 140 outputs the signals based on data in the storage device 100b, such as the CD, using at least two independent audio channels. A second amplifier 150 may amplify the outputted signals and speakers 160a and 160b convert the amplified signals to sound waves.
Although at least two independent audio channels have substantially identical structure, a signal to noise ratio (SNR) may be degraded by interference between the class-D channels when noises occur in the audio band of 20 Hz to 200 kHz.